Conventional current measurements involve measuring the voltage drop across a resistor. With this measurement technique, single electrical charge carriers cannot be detected because their signals disappear in the noise.
Low current measurement by electron counting has already been achieved at cryogenic temperature using a one dimensional series array of tunnel junctions combined with a radio-frequency single-electron transistor, as described in the paper by J. Bylander, T. Duty and P. Delsing, entitled “Current measurement by real-time counting of single electrons”, published in Nature, 434, pp. 361-363 (London, 2005). However, the need for cryogenic temperatures makes this method unpractical.
U.S. Pat. No. 4,370,615 discloses a high-impedance, fast electrometer for measuring small currents.
It is known to use avalanche transistor circuits to generate fast pulses with short rise or fall times and large amplitudes exceeding 10 V. U.S. Pat. No. 3,223,854 discloses such a circuit that decreases the recovery time without decreasing the amplitude of the output pulse of the avalanche transistor circuit thereby increasing the repetition rate at which such circuits can be operated. The circuit is an improvement over the conventional common emitter avalanche transistor circuit for pulse generation. These applications require that the avalanche transistor be of a relatively large size.
European Patent No. EP 1679749 discloses a photodiode using the avalanche effect that is capable of detecting single photons. The photodiode comprises a quenching and recharging circuit allowing its operation in the Geiger mode.